Yesterday's bombing at the United Nations headquarters in Baghdad is a bloody reminder that the epicenter of the global war on terror is now Iraq. This isn't new, but it's taken on new meaning as increasingly desperate terrorists realize just what's at stake.

Sergio Vieira de Mello, the U.N.'s top official in Iraq, put it well in June when he arrived in Baghdad to take up his post. "I have been sent here," he said, "with a mandate to assist the Iraqi people and those responsible for the administration of this land to achieve . . . freedom, the possibility of managing their own destiny and determining their own future." The U.N. envoy was among those killed yesterday.

Mr. Vieira de Mello's vision of Iraqis running a free Iraq is global terrorism's worst nightmare. And it explains why American defeat in Iraq has become its top terrorist priority. Yesterday's bombing was notable in that it wasn't against U.S. soldiers but against non-American civilians trying to restore normalcy to post-Saddam Iraq.

As a "soft-target" the U.N. was of course more vulnerable to a truck bomb. But the U.N. was no doubt also chosen to intimidate countries that are now contemplating the dispatch of either troops or civilian experts to help rebuild Iraq. The persistent attacks on water mains, oil pipelines and other essential infrastructure are also designed to prevent stability from returning to Baghdad.

In an important sense, of course, this is merely validating what some of us have said all along about the war in Iraq. The link between Saddam and al Qaeda might not have been provable beyond a reasonable doubt, but they shared the common purpose of ousting the U.S. from the Middle East. Now the foreign jihadis flooding the country are proving the point by joining up with Baath Party remnants that want to restore Saddam's terrorist rule.

We don't know yet who was responsible for yesterday's attack, but one possibility is Ansar al Islam. This is the al Qaeda-linked group that was rooted out of northern Iraq last March and is now making a comeback. Ansar's signature is truck bombs of the ilk that exploded yesterday and on August 7 at the Jordanian embassy in Baghdad.

The head of the Kurdish security forces in the north was quoted as saying a few days ago that Ansar al Islam followers have been sneaking across the border with Iran and are setting up cells in Baghdad. Iraq has become a "terrorist magnet" for foreign fighters, the commander of the U.S. ground forces in Iraq also said last month. American troops have found numerous foreign passports--from Saudi Arabia, among other countries--on the bodies of terrorists killed on some of their raids.

While this terror threat can't be ignored, it also shouldn't be overestimated. The terrorists lack a superpower patron, as well as a foreign sanctuary. The latter point puts a premium on U.S. efforts to pressure Syria and Iran not to provide such a foreign safehouse. Before yesterday's bombing, Baghdad's news of the day was L. Paul Bremer's statement to an Arabic newspaper that Syria was allowing "foreign terrorists" to sneak across the border. He also expressed concern about Iran's "irresponsible conduct" in meddling in Iraq's affairs.

It didn't help that hours later Mr. Bremer found himself at least partly contradicted in Washington. President Bashar Assad's government was making "limited progress" in restraining terrorists from crossing the border, State Department spokesman Richard Boucher said. If you were Assad, which American would you listen to? Sending mixed messages is exactly the wrong way to get Syria to cooperate.

It's better news that the U.S. is accelerating plans for Iraqi forces to start security patrols. The 7,000-man Iraqi civil-defense force will patrol with U.S. troops at first but then work independently. The new Iraqi militiamen--who know the language and the terrain better than U.S. soldiers--will be especially helpful in identifying foreign fighters. We only wish this step to involve Iraqis in defending their own country had begun months before, as the Pentagon had proposed but the U.S. State Department resisted.

We think that the American people have understood all along that winning in Iraq is crucial to winning the war on terror. "You can't separate the global war on terrorism from what is happening here in Iraq," Centcom commander General John Abizaid told us last month, in words his commander-in-chief could well borrow. "If we can't be successful here, then we won't be successful in the global war on terror. It is going to be hard. It is going to be long and sometimes bloody, but we just have to stick with it."